The Typical Military Bridge

The typical military bridge, therefore, since it must be erected in a short time and without elaborate plant, will be characterized, as a rule, by shallow foundations, by a relatively large number of piers or supports with correspondingly short spans, and by structural members of small size, light weight, and great simplicity. If we add that timber is the material most frequently employed, this delineation of the typical military structure will be recognized as a description of the short span, framed-trestle-and-stringer, timber bridge; and, in fact, this is the type employed in the majority of cases in military practice. If we now permit the occasional use of pile trestles instead of framed trestles, I-beams in place of wooden stringers, and simple wooden or sectional steel trusses for greater spans, and include the standard floating equipage, we shall have enabled the military bridge builder to meet nearly all situations with which he will be confronted.

Some of the economic features of the more usual types of military bridges will now be considered.

Framed Trestles

The framed trestle requires no plant, other than simple tackle, for its erection. It is readily constructed from a great variety of materials, and by unskilled labor; and it meets the majority of emergency situations. The framed trestle, as we have seen, is accordingly the favorite type of support in hasty military bridging in the combat zone. The usual form of bent is the simple, one-plane type with cap, sill, two or more posts or legs, and diagonal sway-bracing.

The trestle will be stiffer against lateral stresses, if the outer posts be inclined or battered; but with unskilled labor it is easier to make all the posts vertical. If pieces of sufficient length are available, the stiffness may be greatly increased by extending both the cap and the sill a foot or more beyond the outer posts and attaching the sway braces to the ends of the cap and sill as well as to each post.

If the depth of water is such that the trestles tend to float up, the bottoms of the posts may be boxed in and the compartments filled with stone.

If the river bottom be of low bearing power, a sill of greater width than that of the posts may be employed. The bearing power of the bottom may be increased by brush mattresses or fascines, wooden mud-sills (where the water is shallow), or rip-rap. It is usually well worth while to resort to such measures, in order to avoid the use of piles.

Economic Span of Trestles

The proper economic span of pile or framed trestles is determined by balancing the time and material required for the bents against that required for the stringers. The problem cannot be solved with mathematical precision. It  depends  upon  the  height  of  the trestles, the difficulty of placing